Pseudo noise codes, sometimes referred to as pseudo random binary sources (of the type derived from feedback shift registers), are currently being adopted for two way ranging systems. In two way ranging systems a pseudo noise code is transmitted from a base station to a remote station, including a transponder, which retransmits a pseudo noise code back to the base station. At the base station, the code transmitted from the transponder is coherently detected by a matched filter and the phase difference between the transmitted and received codes is detected to determine the range between the stations. Because of bandwidth limitations, it is desirable to transmit information, represented by binary signals derived asynchronously with the codes, simultaneously with and in the same frequency range as carriers for the pseudo noise codes. One technique that has been proposed involves quadriphase, digital modulation of a suppressed carrier by a pair of pseudo noise codes and a pair of sources of binary data. In particular, the modulo two sum of a first pseudo noise code and a first binary signal is formed to biphase modulate a first component of a carrier. The modulo two sum of the second pseudo noise code and the second data source biphase modulates an orthogonal component of the carrier. The two biphase, orthogonal components are linearly combined to form the digitally, quadriphase modulated suppressed carrier.
The proposed technique can be used only in connection with binary data sources having a bit rate that is no greater than approximately one tenth the chipping rate of the pseudo noise codes, i.e., the bit rate can be no greater than of the order of the chipping rate. (The chipping rate of a pseudo noise code is the rate at which the individual pulses in the pseudo noise code stream are derived and therefore is equal to the code repetition rate divided by the number of members in the code; one pulse of the noise code is referred to as a chip.) Otherwise, intersymbol interference between the data bits and PN code occurs and it is not possible to separate the data bits from the PN chips.
Two techniques have been considered to solve the problem of transmitting asynchronous relatively high frequency binary signals simultaneously with transmission of coherent two way pseudo noise codes. In accordance with one of the techniques, a transmitter at the transponder has a telemetry rate synchronized to the pseudo noise code received by the transponder. This technique has the disadvantage of restricting the telemetry bit rate transmitted from the source. To enable the telemetry rate to be synchronized with the received pseudo noise code, it is necessary to store the asynchronously occurring binary data bits so that they can be transmitted at the same bit rate as the received chip rate. Storing the binary data bits has the primary disadvantage of destroying the time synchronization of the data relative to the remote station clock and the additional disadvantage of increased hardware; also, in certain instances it may not be possible to transmit all of the data bits that are derived from the binary sources.
Another proposed solution to the problem involves synchronizing the returned pseudo noise code chipping rate to the data bit rate with a subcarrier modulated with information containing the relative time difference between the pseudo noise codes received and transmitted from the transponder. This technique is not considered to be adequate because the remote station must make a one way range measurement which is limited in accuracy by the relative time error between the base station clock and the remote station clock. Also, the technique requires continuous monitoring of the relative time between received and transmitted pseudo noise codes at the transponder, thereby increasing the hardware and software complexity at the remote station. A further disadvantage is that non-standardized range measurements must be employed at the base station, thereby increasing the cost of the base station apparatus and software for determining the transponder range. A further disadvantage is that the base station requires additional equipment to detect the relative time difference inserted by the transmitter on the subcarrier.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of telemetering range indicating pseudo noise codes, as well as binary data having a bit rate at least on the order of magnitude of the pseudo noise chipping rate.
Another object of the invention is to provide a new and improved apparatus for and method of simultaneously telemetering, in the same frequency band, pseudo noise ranging codes and binary data.
An additional object of the invention is to provide a telemetering apparatus and method wherein pseudo noise ranging codes are modulated with a relatively low bit rate data stream that is independent of a high bit rate data stream telemetered simultaneously with and in the same frequency band as the ranging codes.